Allow me to shed some light on that comment and hopefully answer your question.
Obesity is a matter of calories. Calories come from many sources—fat, sugar, and energy-dense products or ingredients. It involves the overconsumption of calories over time. Some people who are obese consume no soft drinks. Some people who are obese consume some soft drinks. Some very thin people—from a BMI perspective—consume a lot of soft drinks.
The challenge in working through obesity is that it's a complex issue. It is multifaceted. It is possible to say that overconsumption of calories over a period of time leads to obesity, but it is not possible to identify any one food or beverage. Diets are different, physical activities are different, and sources of calories are different. It is possible to become obese by drinking nothing but milk, which most folks would consider one of the healthful beverages out there.
I hope I've answered your question. As to the science behind causally linking one food or beverage to obesity, I think I've been fairly transparent. I've said that you will find some studies that will suggest that there is a link. You will also find a wide variety of studies that suggest there is no link. What I'm saying is that no causal link has yet been established. To establish that link, further scientific work is required to make this link clear from a preponderance of the evidence.